Vibration is a problem which is freqently encountered in the operation of helicopters. An oscillatory hubshear force is produced, for example, at the rotor hub of the lift unit of a two-bladed helicopter with the oscillatory force having a frequency that is two times that of the revolution rate of the rotor, i.e., two-per-rev. This force is primarily generated by the effects of the translational speed of the helicopter and by the periodic pitch change of each rotor blade during each revolution. This hubshear force is generated in the blades and transmitted vertically through the rotor mast to the helicopter fuselage. Excessive vibration resulting from this periodic force received at the fuselage causes discomfort for the passengers and decreases the life of fatigue-loaded components. Efforts have been made and continue to be made to attenuate or eliminate these vibrations in order to extend the operational life of the aircraft and improve the ride characteristics.
The problem of hubshear vibrations in a helicopter rotor have been addressed in U.S. Pat. No. 3,540,809 to Paul, et al. In this patent, it is shown that inplane rotor vibrations can be absorbed to some extent by means of bifilar vibration dampers which rotate with the helicopter rotor. The bifilar dampers oscillate while rotating with the mast and are tuned to absorb vibrations generated at a particular frequency. Further helicopter vibration absorber devices utilizing pendulums are disclosed in U.S. Pat. No. 3,372,758 to Jenney and U.S. Pat. No. 3,053,643 to Kelley, et al.
Despite the development of the above vibration absorbers, there exists a need for a grip or blade mounted, tunable, free pendulum for damping out-of-plane vibrations in a helicopter rotor.